As explained in co-pending application U.S. Ser. No. 814,591, it is known to combine latent photoacids such as esters of diazonaphthoquinone sulfonic acids with alkali soluble resins such as the well-known novolacs (phenol-formaldehyde condensation products) to form positive photoresist materials. The novolacs are soluble in aqueous alkaline solutions and consequently can be removed during the developing process. However, when the latent photoacid is an insoluble material, it prevents the composition from dissolving until exposed to actinic radiation. At that time, the photoacid becomes soluble in alkali, as in the novolac resin, so that the composition can be removed from those portions of a surface which have been exposed to radiation.
In recent years, there has been increased interest in using deep ultraviolet (DUV) radiation in order to improve the sharpness of the developed image, permitting smaller features to be reproduced. Unfortunately, the novolac polymers are not suitable for this use since they absorb significant amounts of DUV radiation and prevent the proper development of the desired image. Thus, materials which do not absorb DUV radiation have been sought. One example of such a product is the copolymer of maleimide and comonomers such as substituted styrenes as shown in U.S. Ser. No. 814,591. While such materials are relatively much more transparent to DUV radiation than are the novolacs, nevertheless there are certain disadvantages which have not been overcome with such compositions and further improvement is desirable. An improved polymer is shown in U.S. Ser. No. 829,874 in which the styrenes are replaced by various vinyl ethers or esters. Still further improvements have been sought.
Various workers in the field have described polymers which contained certain degradable groups, sometimes called "blocking groups", which permit the polymers' characteristics to be changed when acid is produced by irradiating the sensitizer (latent photoacid). If, for example, a blocking group is placed on a novolac resin, which in itself is soluble in aqueous alkaline developer solution, it can be transformed into an aqueous insoluble resin which resists the alkaline solution except in those areas in which the latent photoacid has been exposed, thereby releasing acid to effectively deblock (i.e., separate) the blocking groups from the novolac composition, so that it reverts to its normal alkaline soluble form. Such a procedure is disclosed by Smith et al. in U.S. Pat. No. 3,779,778. Ito et al. in U.S. Pat. No. 4,491,628 show a similar protolytic technique applied to a different polymer, a poly(hydroxystyrene).
In U.S. Ser. No. 832,116, blocking groups are applied to the maleimide-styrene or maleimide-vinylester or ether polymers of U.S. Ser. Nos. 814,591 and 829,874 in order to improve their performance. With regard to the polymer of U.S. Ser. No. 832,116, it has been found that appreciably better results are obtained than with the compositions shown in U.S. Ser. No. 814,591. However, the deblocking of the maleimide-styrene polymer requires precise control of the deblocking conditions, which takes place at relatively high temperatures. It would be preferable to have a material which is both capable of being deblocked at relatively moderate temperatures after the photoacid had been exposed to radiation, and requires less precise control of the deblocking conditions.
In U.S. Pat. No. 4,101,323 Buhr et al., teaches another aspect of the broad principle, i.e., removing blocking groups with acid generated by radiation exposure of a latent photoacid. Buhr's compound 76 could appear to be similar to the invention to be discussed below, but the blocking group of Buhr et al. would be split off entirely to form the soluble imide compound and, consequently, does not suggest the two-stage deblocking characteristic of the present invention. Also, Buhr's compound is not incorporated in a polymer and is intended for use in copying compositions.
A need remains for an improved photoresist composition, especially in the DUV region, which has better processing characteristics and produces the finer patterns which are desired.